Not to mention that the adoption of a new weapon without thinking through the tactical and strategic changes it brings usually invokes the Law of Unexpected Results and Unintended Consequences. One reason why the Civil War was such a slaughterhouse was the tactics were based on the smoothbore with its 100 yard effective range but the troops were using rifled muskets with a 4-500 yard effective range.
In WWII the Soviets fielded much greater numbers of SMGs-the PPSh M1941
and PPS M1943-than any other army, they were easy to train conscript troops with and their short range encouraged aggresive tactics, plus Soviet troops were usually supported with massive ammounts of artillery.